Shedding of a pair of sessile droplets

Abstract

Droplets detachment from a solid surface has many applications including icing prevention in aircrafts and offshore structures, or water management in fuel cells. A droplet which is placed on a surface and is exposed to an airflow, can be shed, if the drag force overcomes the droplet's adhesion force to the surface. This study elucidates how the onset of the shedding of a sessile droplet can be influenced by presence of other droplets in its vicinity. Presence of another sessile droplet, in proximity, changes the airflow pattern, so the drag force, and consequently shedding of the both droplets will be affected. This experimental study uses a pair of 5 and 10 μl water droplets placed either in tandem or in a side-by-side arrangement, within a laminar airflow. The critical air velocity (Ucr) at which the upstream droplet/droplets shed, depends on the arrangement type as well as the spacing of droplets in each arrangement. For both arrangements, the spacing range within which droplets are interacting, and the interaction effects on Ucr, was studied. For droplets arranged in tandem, the critical air velocity of the upstream droplet, starting from the value greater than Ucr of a single droplet, decreases with an increase in spacing. On the other hand, for the side-by-side arrangement, the maximum increase in Ucr is observed at an intermediate spacing. The results show that the wettability of the surface, does not affect the general trend of the Ucr with spacing; but for hydrophobic surfaces, the range of spacing within which the droplets interact, becomes narrower. Finally, an empirical model is proposed to predict the Ucr based on droplet spacing, and arrangement.